So, what if full bard (Minstel, I guess) was a special class?
My idea is unlocking unique mechanics. For example, for casting skills you'd have to play sequence of notes. It could be something like
Guitar Hero or
Magicka's spell casting or Invoker from Dota 2 mechanics. Well, that's the simple way.
As mages can specialize in elements, Minstel could specialize in scales. Major scales would be buffs and minor scales debuffs, something like that. You would have to keep playing notes of certain scale for a certain buff, but playing accordingly to music harmony and music theory stuff. That would reward real life musicians and players that like fast reaction based mechanics.
Or maybe you could be able to compose songs by crafting the notes? Imagine your clan listening to their favorite songs irl while being buffed!
And for those who don't know much about music, Minstel quests could be something about learning it.
Ok, I know it's probably not going to happen and it would probably not work, but I'd love to play as a bard like that.